A four-year study recently published in the journal Weed Science shows how far-reaching the impact of a single herbicide-resistant weed can be.

The research was conducted over four years in four Arkansas cotton fields. In this study, 20,000 seeds of glyphosate-resistant Palmer amaranth, which may represent only 2 percent of seed from one plant, were introduced into a 1-square-mile area. The weeds that resulted were not managed, but allowed to “escape.”

In test fields, the seeds of one mature glyphosate-resistant Palmer amaranth plant were released. In the third year of crop production following the release, complete crop failure occurred due to infestation of this weed. This study shows the need for a zero-tolerance threshold in the management of glyphosate-resistant Palmer amaranth.

Seeds can be dispersed by wind and water, and moved by animals, humans and machinery. Weeds such as Palmer amaranth, that can produce a large amount of small seeds capable of floating in water, can spread rapidly throughout a production field. Palmer amaranth has prolific seed production, rapid dispersal and high competitiveness with crops, making herbicide-resistant strains difficult to control.

In the Arkansas study, glyphosate herbicide was the only weed management used. In the first growing season, a separate patch of Palmer amaranth emerged 375 feet from the original location. In the second year, resistant plants expanded to reach field boundaries and infested 20 percent of the field area resulting in decreased yield and significant problems with cotton harvest. By the third growing season, glyphosate-resistant Palmer amaranth had completely colonized the fields, making the cotton crop impossible to harvest.

The expansion of resistant weeds seen in this research helps to explain the rapid takeover of many farms by glyphosate-resistant Palmer amaranth, particularly when glyphosate was the only means of weed control. It also demonstrates the need to keep all resistant-prone weeds from escaping control to prevent loss of an herbicide or technology. Weed control based on an economic threshold (dollars spent versus dollars returned) does not adequately consider the soil seedbank and the risk for herbicide resistance, according to the study.